Atomic configurations of dislocation core and twin boundaries in 3C-SiC studied by high-resolution electron microscopy

The defects in 3C-SiC film grown on (001) plane of Si substrate were studied using a 200 kV high-resolution electron microscope with point resolution of 0.2 nm. A posterior image processing technique, the image deconvolution, was utilized in combination with the image contrast analysis to distinguish atoms of Si from C distant from each other by 0.109 nm in the [110] projected image. The principle of the image processing technique utilized and the related image contrast theory is briefly presented. The procedures of transforming an experimental image that does not reflect the crystal structure intuitively into the structure map and of identifying Si and C atoms from the map are described. The atomic configurations for a 30 degrees partial dislocation and a microtwin have been derived at atomic level. It has been determined that the 30 degrees partial dislocation terminates in C atom and the segment of microtwin is sandwiched between two 180 degrees rotation twins. The corresponding stacking sequences are derived and atomic models are constructed according to the restored structure maps for both the 30 degrees partial dislocation and microtwin. Images were simulated based on the two models to affirm the above-mentioned results.

High resolution electron microscopy study of single chain single crystals of gutta percha

Single chain single crystals (SCSC) of gutta percha (GP) were prepared by a dilute-solution spraying method. Electron diffraction (ED) patterns revealed that the single chain single crystal was of a new crystalline modification, the delta form. The images of SCSC of GP obtained with a high resolution electron microscope (HREM) showed a two dimensional periodic structure. Most of the images consisted of lattice fringes derived from the (001) zone. This is the first time that the single chain single crystal images of GP have been observed at a molecular level. Micrographs were image processed using optical filtering methods to improve the signal-to-noise ratio, and were compared with computer-generated simulations of the images. From the viewpoint of the defects seen in high resolution images, the crystal formation and melting processes are discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.

INVESTIGATION OF THE GROWN DIAMOND (100) SURFACE USING HIGH-RESOLUTION ELECTRON-ENERGY LOSS SPECTROSCOPY

The diamond (100) facets deposited at initial 1.0% CH4 have been investigated using high resolution electron energy loss spectroscopy (HREELS). The diamond (100) facets grown at 800-degrees-C are terminated by CH2 radicals, and there is no detectable frequency shift compared with the characteristic frequencies of molecular subgroup CH2. Beside the CH2 vibration loss, CH bend loss (at 140 meV) of locally monohydrogenated dimer is detected for the diamond (100) facets grown at 1000-degrees-C. Dosing the (100) facets grown at 800-degrees-C with atomic hydrogen at 1*10(-6) mbar, the loss peak at 140 meV appears. It is suggested that there are enough separately vacant sites and uniformly dispersed monohydrogenated dimers on (100) facets. This structure relaxes the steric repulsion between the adjacent hydrogen atoms during the diamond (100) surface growth.

Microstructure and Evolution of Mechanically-Induced Ultrafine Grain in Surface Layer of Al-Alloy Subjected to USSP

Experiments were conducted to investigate the ultrafine-grained (UFG) microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening. Conventional and high-resolution electron microscopy was performed at various depths of the deformed layer. Results showed that UFG structures were introdued into the surface layer of 62 μm thick. With increasing strain, the various microstructural features, e.g., the dislocation emission source, elongated microbands, dislocation cells, dislocation cell blocks, equiaxed submicro-, and nano-crystal grains etc., were successively produced. The grain subdivision into the subgrains was found to be the main mechanism responsible for grain refinement. The simultaneous evolution of high boundary misorientations was ascribed to the subgrain boundary rotation for accommodating further strains. Formed microstructures were highly nonequilibratory.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Localized Solid-State Amorphization at Grain Boundaries in a Nanocrystalline Al Solid Solution Subjected to Surface Mechanical Attrition

Using high-resolution electron microscopy, localized solid-state amorphization (SSA) was observed in a nanocrystalline (NC) Al solid solution (weight per cent 4.2 Cu, 0.3 Mn, the rest being Al) subjected to a surface mechanical attrition treatment. It was found that the deformation-induced SSA may occur at the grain boundary (GB) where either the high density dislocations or dislocation complexes are present. It is suggested that lattice instability due to elastic distortion within the dislocation core region plays a significant role in the initiation of the localized SSA at defective sites. Meanwhile, the GB of severely deformed NC grains exhibits a continuously varying atomic structure in such a way that while most of the GB is ordered but reveals corrugated configurations, localized amorphization may occur along the same GB.

Plastic Deformation Of Nanocrystalline Nickel

A high-resolution electron microscopy study has uncovered the plastic behavior of accommodating large strains in nanocrystalline (NC) Ni subject to cold rolling at liquid nitrogen temperature. The activation of grain-boundary-mediated-plasticity is evidenced in NC-Ni, including twinning and formation of stacking fault via partial dislocation slips from the grain boundary. The formation and storage of 60A degrees full dislocations are observed inside NC-grains. The grain/twin boundaries act as the barriers of dislocation slips, leading to dislocation pile-up, severe lattice distortion, and formation of sub-grain boundary. The vicinity of grain/twin boundary is where defects preferentially accumulate and likely the favorable place for onset of plastic deformation. The present results indicate the heterogeneous and multiple natures of accommodating plastic strains in NC-grains.

Crystallization of amorphous Si films by pulsed laser annealing and their structural characteristics

Nanocrystalline silicon (nc-Si) films were prepared by pulsed laser annealed crystallization of amorphous silicon (alpha-Si) films on SiO2-coated quartz or glass substrates. The effect of laser energy density on structural characteristics of nc-Si films was investigated. The Ni-induced crystallization of the a-Si films was also discussed. The surface morphology and microstructure of these films were characterized by scanning electron microscopy, high-resolution electron microscopy, atomic force microscopy and Raman scattering spectroscopy. The results show that not only can the alpha-Si films be crystallized by the laser annealing technique, but also the size of Si nanocrystallites can be controlled by varying the laser energy density. Their average size is about 4-6 nm. We present a surface tension and interface strain model used for describing the laser annealed crystallization of the alpha-Si films. The doping of Ni atoms may effectively reduce the threshold value of laser energy density to crystallize the alpha-Si films, and the flocculent-like Si nanostructures could be formed by Ni-induced crystallization of the alpha-Si films.

Alloy compositional fluctuation in InAlGaN epitaxial films

The quaternary InAlGaN films were grown by metal-organic vapor phase epitaxy (MOVPE) at various temperatures and the optical and structural properties of the quaternary films were investigated by temperature-dependent photoluminescence (PL), high-resolution X-ray diffraction (HRXRD) and high-resolution electron microscopy (HREM). The results show that the temperature-dependent PL intensity of the InAlGaN film is similar to that of the disordered alloys, which is thought to be due to local alloy compositional fluctuations (ACF) in the epilayer. HRXRD measurement reveals there are In-rich and In-poor phases in the film and HREM observation, on the other hand, demonstrates that nanoclusters formed in the epilayer. Therefore the experimental results support the existence of ACF in the epilayers.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Influence of ion energy and deposition temperature on the surface morphology of carbon films deposited by ion beams

Carbon films were deposited by mass-selected ion beam technique with ion energies 50-200eV at a substrate temperature from room temperature to 80 degreesC,. For the energies used, smooth diamond-like carbon films were deposited at room temperature. When the substrate temperature was 600 degreesC,rough graphitic films were produced. But highly oriented carbon tubes were observed when the energies were larger than 140eV at 800 degreesC. They were perpendicular to the surface and parallel to each other. preferred orientation of graphite basic plane was observed by high-resolution electron microscopy. Shallow ion implantation and stress are responsible for this orientation.

TEM study of dislocations in ZnTe/GaAs heterostructure grown by hot-wall epitaxy

A ZnTe layer grown on GaAs substrate by hot-wall epitaxy (HWE) was studied using transmission electron microscopy (TEM). For a (110) cross-sectional specimen, its (001) ZnTe/GaAs interface was analysed by large angle stereo-projection (LASP) and high resolution electron microscopy (HREM). In the LASP, a double diffraction occurred and moire fringes were formed, meanwhile misfit dislocations were revealled clearly by weak beam technique. In HREM, not only Lomer and 60 degrees types of misfit dislocations were observed, but also two types of stacking faults were analysed. The residual strain was estimated by both methods.

Growth of near planar Si0.5Ge0.5 epitaxial layers directly on Si substrate by UHV/CVD at 500℃

The relationship between Ge content of Si1-xGex layers and growth conditions was investigated via UHV/CVD system at relative low temperature of 500℃. Si1-xGex layers were in a metastable state in this case. 10-period strained 3.0 nm- Si0.5Ge0.5/3.4 nm- Si multi quantum wells were obtained directly on Si substrate. Raman Measurement, high resolution electron microscopy and photoluminescence were used to characterize the structural and optical properties. It is found that such relative thick Si0.5Ge0.5/Si multi quantum wells are still near planar and free of dislocations, that makes it exploit applications to electrical and optical devices.

Growth and photoluminescence of InAlGaN films

Two quaternary InAlGaN films were grown by metal-organic chemical-vapor deposition (MOCVD) on sapphire (0001) substrates with and without high-temperature GaN interlayer, respectively. The structural and optical properties of the quaternary films were investigated by high-resolution X-ray diffraction (HRXRD), high-resolution electron microscopy (HREM), temperature-dependent photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy. According to the HRXRD and PL results, it is demonstrated that two samples have the same crystal quality. The TRPL signals of both samples were fitted well as a stretched exponential decay from 14 K to 250 K, indicating significant disorder in the materials, which is attributed to recombination of excitons localized in disorder quantum nanostructures such as quantum dots or quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-section HREM measurement further proves that there exist disorder quantum nanostructures in the quaternary. By investigating the temperature dependence of the dispersive exponent beta, it is shown that the stretched exponential decays of the two samples originate from different mechanisms. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Structure image of single crystal of polyethylene

High-resolution electron macroscopic images of lamellar single crystal of polyethylene (PE) have been successfully obtained using high-resolution electron microscopy (HREM), although so far the feasibility of obtaining HREM images from such a radiation sensitive polymer is still drastically questioned. The HREM images with a clear two-dimensional periodic structure reported here were recorded in a transmission electron microscope operated at 200 kV. The images consisted of lattice fringes derived from the <001> zone, and the structure images of different lattice fringes were resolved. To our knowledge, this is the first time that such clear structure images of PE have been reported at a molecular level.

Quantitative strain characterization of SiGe heterostructures by high-resolution transmission electron microscopy

We report the quantitative strain characterization in semiconductor heterostructures of silicon-germaniums (Si(0.76)Geo(0.24)) grown on Si substrate by an ultra-high vacuum chemical vapor deposition system. The relaxed SiGe virtual substrate has been achieved by thermal annealing of the SiGe film with an inserted Ge layer. Strain analysis was performed using a combination of high-resolution transmission electron microscopy and geometric phase analysis.